P3A 100W layout - comments?

[Vostro]

Maybe mount Q5,6,9 inline with each other on a small alu plate,
Then outputs on the big heatsink.

[Stijnio]

Okay, so it should track Q5. I will make the adjustments in the PCB today.

I know the board can be a lot smaller, but I want to use heatsinks that I already have from an older amplifier wich already have screw holes 11cm from each other. Do you think the width of the board will decrease performance? (noise etc.)

[sreten]

Hi,

The bias loop is Q9, Q5 and Q6. Mounting Q9 on the heat sink wont help
at all in matching Q5 and Q6. Q7 and Q8 temperatures do not affect bias,
unless they are thermally linked to Q9.

Cheapest and simplest is to mount them all on the same heat sink (bracket).

Alternatively Q5 and Q6 have their own or shared small heatsink,
here Q9 should be linked to at least one of the devices, in theory.

This is all theoretical, and the P3A is very liberal in its claimed biasing
requirements, whilst in fact the optimum bias is a very precise affair.
(See Douglas Self and CFP output stage optimal bias.)

If your planning on a nominal 50mA to 100mA bias its already
overbiased and most of the above does not matter. Overbias
can wander around without becoming any sort of real issue.

Just make sure Q5 and Q6 have heatsinks.

rgds, sreten.

[Stijnio]

Thank you sreten for the great explanation! I will use a 75mA bias current, but to be sure I've put Q9 near Q5.
I have also added an extra Cap parellel to R5.




Any comments to improve my design?

[odysseybmx414]

Try to place the feedback takeoff point (R5) closer to the center of the output track, best if it is right there at the connector for the speaker wire.

[Stijnio]

Okay! I moved R5 closer to X1 where the speaker is connected.



Any recommendations?

[donpetru]

1. Are scattered electronics on the PCB. You must place components on PCB more economical.
2. You want to build one or two channels? If you want to build two channels or even one, it would be good to include the PCB and power supply, so that you can manage better and correct way PCB routing traces.
3. Some transistors should be mounted on the heatsink, like Q4, Q5 and Q6, not only Q7 and Q8.
4. Emitter of Q1 and Q2 connected directly to the collector of the transistor Q3 is not the best choice. Why? Eg. when you connect the input signal, a simple mishandled it, can cause a pretty annoying noise in the speaker. To avoid this, we need some resistors there, perhaps a trimmer to help dc offset adjustment's..., even if you want to build a simple amplifier. So, we should not overlook that part.

[Stijnio]

Thank you for your reply Donpertru.

I have some questions about your recommendations.
1. Wich components should I group?

2. Do you mean that I should put the layout of the power supply in this topic?

3. Q4, Q5, Q6 will be equipped with a heatsink.

4. What value resistor do you advise there?

[donpetru]

1. Almost all electronic components must be re-placed on the circuit board. Look - for example - make a comparison between one of the PCB's designed by me and attached below, and your PCB, to see how well you placed components.

2. Yeah, I do not mean only power supply electronic diagram, but the inclusion of that diagram in the PCB power amplifier.

3. Yes, you will need to attach a small heatsink those transistors.

4. Two resitors 22 Ohm 1% and one resistor about 4.7k.

[Stijnio]

I have made some adjustments to the PCB. At first, the PCB its much smaller now. It was difficult to reposition the component because the two power transistors have a fixed position due the heatsink.

Also I have added two 100uF caps at the +35V and -35V (C9, C10)